Which ahs the best multiplayer experience still?

Probably Ureal Tournament 2004 for the fact that it may have a more active community out of all of them, but I could be wrong. I've only really played Ureal Tournament 99 so I can't say much. but If you want me to I could refer you to an active group that has an active server for the game.

Both. Both versions have tons of mods etc. that don’t necessarily exist for the other version. E. g. Infiltration 2.9 for UT 99 is second to none in terms of realistic weapons etc. And the bots and the original UT 99 maps are worth trying.
I love both versions. If you don’t find real players for UT 99, the bots will be able to keep you busy. ;)

Read the differences, there is a lot of reviews about that. UT99, UT2004 no matter, both are legendary masterpieces and probably the most well balanced FPS game, not to mention the still active playerbase. The only thing that matter is your taste about the core gameplay. I prefer UT2004 because of the vehicles, and that's my taste. :)

UT99 is somehow harder to master than UT2004, at least I feel like that. Don't take me wrong UT2004 has a lot of place to come out with new tricks and such, but the weapons are closer to today's standard.

Actually the games are cheap yet their gameplay-value is over the top. Eventually you should get both games whenever you catch a discount. About the order, I suggest you something. If you're a hardcore FPS fan and really into multiplayer and community level duels, pick UT99 first. If you want to have fun by yourself or playing multiplayer matches with friends without serious take-on, pick UT2004.

both are still worth playing really. both are still heads and tails above other games in the genre. And fortunately, both have enough people playing you could get a game or two here and there that is worth your time.

I do admit I play more 2004 over 99. but 99 still has face, and that is mostly what I play in 2004. Best map ever :) (just don't play face 3 in 2004, face classic is way better)